1. Field of the Invention
The present invention relates to data transmission systems, and, in particular, to equalizer-based receivers.
2. Description of the Related Art
Code-Division Multiple-Access (CDMA) systems allow many users simultaneously to access a given frequency allocation. User separation at the receiver is possible because each user spreads its respective modulated data waveform over a wide bandwidth using a unique spreading code, prior to transmitting the waveform. Such spreading typically involves, e.g., multiplying the data waveform with a user-unique high-bandwidth pseudo-noise binary sequence. At the receiving end, the receiver re-multiplies the signal with the pseudo-noise binary sequence to remove substantially all of the pseudo-noise signal, so that the remaining portion of the signal is just the original data waveform. Ordinarily, users spread their signals using codes that are orthogonal to each other, i.e., do not interfere with one another. However, a common problem is inter-symbol interference (ISI), i.e., distortion of a received signal typically manifested in the temporal spreading and consequent overlap of individual pulses from nearby users to the degree that a receiver cannot reliably distinguish between changes of state representing individual signal elements. ISI can present a significant problem if the power level of the desired signal is significantly lower than the power level of the interfering user (e.g., due to distance) and, at a certain threshold, can compromise the integrity of the received data.
One technique for handling ISI is the use of equalizer-based receivers, which are a promising technology for high-speed data transmission systems, such as High-Speed Downlink Packet Access (HSDPA), a standard that is part of the Third-Generation Partnership Project (3GPP). Equalizer-based receivers typically use linear-channel equalizers to restore the orthogonality of spreading sequences lost in frequency-selective channels, thereby suppressing ISI, such as might occur in a downlink operating under the Wide-Band CDMA (WCDMA) standard (a 3GPP technology). Equalizer-based receivers also have the advantage of being of relatively low complexity for short to moderate signal-delay spreads.
The typical 3GPP HSDPA equalizer-based receiver comprises a multi-tap filter coupled to a delay line of received complex data samples, with each filter tap multiplied by a complex weight, followed by a spread-spectrum demodulator and, optionally, a constellation de-mapper. Equalization, demodulation, and de-mapping involve continuously extracting or generating channel parameters from the received signal, and then using these parameters to process the received signal. An intrinsic problem in equalizer-based receivers is that there are performance-degrading time delays between extraction or generation of processing parameters from the received signal and subsequent application of the parameters to the received signal.
For example, FIG. 1 illustrates an exemplary prior-art equalizer-based receiver 100, which comprises a plurality of processing stages or blocks: an input buffer 101, an equalizer filter 102, a spread-spectrum demodulator 103, and a symbol de-mapper 104. Input buffer 101 constitutes a delay line for received input samples and outputs delayed samples to equalizer filter 102, which outputs a sequence of filtered chips. Demodulator 103 demodulates (e.g., descrambles, despreads, and de-rotates) the filtered chips, resulting in a sequence of symbols that are provided to de-mapper 104, from which de-mapper 104 derives and outputs a set of Log-Likelihood Ratios (LLRS) as the output of equalizer-based receiver 100. Equalizer filter 102, spread-spectrum demodulator 103, and symbol de-mapper 104 will now be described in further detail with reference to FIGS. 2, 3, and 4.
With reference now to FIG. 2, equalizer filter 102 is illustrated. Equalizer filter 102 comprises a pre-equalizer block 105 and a Finite-Impulse Response (FIR) filter 106. Pre-equalizer block 105 receives filtered chips from FIR filter 106, which pre-equalizer block 105 uses to calculate an error measure that serves as the basis for updating one or more filter taps of FIR filter 106 by providing a set of tap weights to FIR filter 106. The tap weights might be generated, e.g., by implementing a Least-Mean-Square (LMS) algorithm, as described in K. Hooli, “Equalization in WCDMA Terminals,” Ph.D. thesis, Department of Electrical and Information Engineering, University of Oulu, Oulu, Finland, 2003, incorporated herein by reference. FIR filter 106 receives input samples from input buffer 101 and the tap weights from pre-equalizer block 105 and uses a set of complex multiply-and-accumulate (MAC) circuits (not shown) and adders (not shown) to produce the filtered chips that are provided to demodulator 103 and pre-equalizer block 105. The FIR filter tap weights are generated in time delay_1, i.e., the tap weights trail the samples to which the tap weights actually correspond by an amount of time delay_1.
Turning now to FIG. 3, demodulator 103 is illustrated. Demodulator 103 comprises a channel estimator 107, a descrambling and despreading block 108, and a de-rotation block 109. Descrambling and despreading block 108 receives the filtered chips provided by equalizer filter 102 and produces a sequence of symbols, which are provided to de-rotation block 109. De-rotation block 109 de-rotates the symbols using channel-estimation parameters provided by channel estimator 107 and outputs the de-rotated symbols to symbol de-mapper 104. Channel estimator 107 receives the filtered chips provided by equalizer filter 102 and produces the channel-estimation parameters. The symbols provided by descrambling and despreading block 108 are generated in time delay_2a, i.e., the symbols being de-rotated trail the chips to which the symbols correspond by an amount of time delay_2a. The channel-estimation parameters provided by channel estimator 107 to de-rotation block 109 are generated in time delay_2b, i.e., the channel-estimation parameters trail the chips to which the channel-estimation parameters correspond by an amount of time delay_2b. Accordingly, the channel-estimation parameters provided to de-rotation block 109 trail the symbols being de-rotated by an amount of time equal to delay_2b minus delay_2a. 
Now referring to FIG. 4, de-mapper 104 is illustrated. De-mapper 104 comprises an energy calculation block 110 and an LLR (or other metric) calculation block 111. Energy calculation block 110 receives the symbols provided by demodulator 103 and uses these symbols to calculate energy parameters that are provided to LLR calculation block 111. LLR calculation block 111 receives the symbols provided by demodulator 103 and uses these symbols, along with the energy parameters provided by energy calculation block 110, to calculate the LLRs that are provided as the output of equalizer-based receiver 100. The energy parameters provided by energy calculation block 110 to LLR calculation block 111 are generated in time delay_3, i.e., the energy parameters trail the symbols to which the energy parameters actually correspond by an amount of time delay_3.
Thus, it can be seen that the parameters that are generated by blocks 102, 103, and 104 (the tap weights, channel-estimation parameters, and energy parameters, respectively) arrive at their respective processing blocks (blocks 106, 109, and 111) later in time than the samples or symbols to which they are applied during processing. The results of the processing suffer due to the “old” parameters being used to process “new” samples or symbols, introducing latency and error into the processing. When taken together, the cumulative effects of delay_1, delay_2a, delay_2b, and delay_3 result in a significant degradation in performance of equalizer-based receiver 100.
Numerous techniques to improve performance of equalizer-based receivers are known in the art, such as those disclosed in S. Qureshi, “Adaptive Equalization,” Processing of IEEE, 1985, incorporated herein by reference, and K. Hooli, “Equalization in WCDMA Terminals,” cited above. Such techniques, however, tend to increase significantly the complexity of the receiver in exchange for only a modest performance improvement. Some of these techniques, e.g., lengthening the filter, introduce side effects that adversely affect performance improvement.